Reoviridae such as inter alia reovirus, which replicate in the cytoplasm, are nonenveloped with an icosahedral symmetry and a double-shelled capsid. The viral genome consists of 10 double-stranded RNA (dsRNA) segments, which can be separated into three size classes: L (large), M (medium), and S (small). Proteins encoded by the genome are also separated into three sizes. The specific genome segments responsible for protein coding have been identified for the S1133 strain of avian reovirus.
Avian reoviruses are members of the orthoreovirus genus in the Reoviridae family. Ubiquitous in commercial poultry, they can be differentiated by antigenic configuration, pathotype, relative pathogenicity, growth in cell culture, embryonated chicken eggs sensitivity to trypsin, and host and tissue specificity.
Classic reoviruses have been isolated from a variety of tissues in chickens affected by assorted disease conditions, including viral arthritis/tenosynovitis, stunting syndrome, respiratory disease, enteric disease, immunosuppression, and malabsorption syndrome. They have frequently been found in chickens that were clinically normal. The nature of the disease that occurs following reovirus infection is very much dependent upon host age, immune status, virus pathotype, and route of exposure. Reoviruses have also been isolated from turkeys, ducks, geese, pigeons, pheasants, parrots, and other exotic avian species.
In young meat-type chickens, economic losses related to reovirus infections are frequently associated with increased mortality, viral arthritis/tenosynovitis, and a general lack of performance including diminished weight gains, poor feed conversions, uneven growth rates, and reduced marketability of affected birds.
Avian Malabsorption Syndrome
Avian Malabsorption Syndrome (MAS) also known as Runting Stunting Syndrome (RSS) is a disease of growing poultry, especially chickens or turkeys, with meat-type or broilers being affected most commonly. This disease has been reported in the Netherlands as Runting and Stunting Syndrome (RSS) but also under different names for example infectious stunting syndrome, enteritis, pale bird syndrome, helicopter disease, infectious proventriculitis, brittle bone disease and femoral head necrosis.
Kouwenhoven et al. (Avian Pathology 17:879-892, (1988)) further defined MAS by five criteria: 1) growth impairment up to 3 weeks after infection of one-day old chicks; 2) excretion of yellow orange mucoid to wet droppings; 3) increased plasma alkaline phosphatase (ALP) activity; 4) decreased plasma carotenoid concentration (PCC); and 5) macroscopically widened epiphyseal growth plates of the proximal tibia. The condition has been further characterized by stunted growth, poor feathering, maldigested feed, lack of skin pigmentation, enteritis, pancreatic atrophy, proventriculitis, thymic and bursal atrophy and bone changes.
The transmission of the disease is effected by oral inoculation of intestinal homogenates from affected chicks into one-day-old broilers. In that experiment, it was demonstrated that low plasma carotenoid levels and elevated ALP activities are suitable tools for the diagnosis of MAS or RSS. In further experiments, MAS was transmitted by oral inoculation of liver homogenates from affected chicks into one-day-old broilers. Despite years of research, the etiology of MAS has not yet been fully established, and the condition is still a major economic problem for the poultry industry. It is believed that viruses are responsible, but bacteria or other microorganisms have not been excluded as causal agents.
Viruses that have been found in birds with outbreaks of MAS or RSS possibly include reoviruses, rotaviruses, parvoviruses, entero-like viruses and a toga-like virus (McNulty and McFerran, 1993, and Pantin-Jackwood MJ- and et al., AVIAN DISEASES 52:235-244, 2008). McNulty (World Poultry 14:57-58 (1998); and Pantin-Jackwood MJ (2008)), summarized the state of art on MAS, and have postulated that identification of the viral causative agent is unknown and recommends control by careful management of production sites.